Deadly earthquakes and tsunamis could be spotted minutes sooner by reading tiny changes in gravity
As many as 10,000 people die each year in earthquakes, with up to 3.1 million quake-related deaths predicted in the coming century. Once an earthquake has been triggered deep in the Earth's crust, each minute that passes is vital to those left unaware in its impact zone.
Now scientists say that watching for changes in gravity could save lives by warning people far earlier of the location and magnitude of big earthquakes.
Current early-warning systems rely on detecting seismic waves, which travel around seven to eight kilometres per second. Gravity signals fire through the atmosphere at more than 300,000km per second, meaning they could be used to catch earthquakes up to 40,000 times faster.
The researchers, from the Paris Institute of Earth Physics, came to their conclusion using data collected during Japan's 2011 Tohoku earthquake. The 9.1 magnitude quake killed more than 15,000 people, and left over 300,000 homeless.
The team looked through seismic data following the quake for a gravitational signal that could have foreshadowed the event. Immediately following the quake, while seismic waves were still on their way to warning stations across Asia, seismographs recorded a gravity change reflective of the event's deadly magnitude, the researchers report. These signals consistently arrived at seismic monitoring stations before their seismic wave counterparts.
If this approach had been available in 2011, the fact the Tohoku quake had a magnitude higher than 9 could have been detected in minutes, the scientists claim. Instead, the near-real-time magnitude provided by the Japan Meteorological Agency was 7.9. This was then corrected three hours later to 8.8, which was another underestimation.
Going forward, the Paris team says these prompt 'elasto-gravity' signals could be used to make earlier estimates of large earthquake magnitudes. The gravity changes or 'elastic waves' generated by earthquakes are a well-known phenomenon.
Last year, a separate group of researchers from the Paris Institute showed it might be possible to detect the size of earthquakes by measuring these elastic waves. 'The 2016 study was able to show evi- dence of a signal. But because this existence was proved through a statistical analysis, it was not exactly the smoking gun we would have hoped for,' Dr. Martin Vallee, lead researcher of the new paper, said. 'In the present study, the early signals related to gravity are observed consistently at sev- eral locations. Additionally, and importantly, we are now able to fully understand the origins of these signals and to model them accurately. Our study finally, concretely shows how these signals are sensitive to earthquake magnitude, which offers a new way to evaluate it very early after the earthquake occurrence.'
The devastating Tohoku-Oki earthquake hit Japan in 2011